The present invention relates to a method and a device for controlling the toner density in copying machines. According to the detection method a toner image corresponding to a simulated original is formed, and the detected density thereof is compared with a reference value. The toner density control device for copying machines maintains optimum toner density regardless of toner color if the copying machine provides colored copies.
There is a known control device for toner density for copying machines (Japanese Laid-Open patent Publication No. 128977/1981) in which a simulated original is attached at a predetermined position away from the setting zone of the original which is on a contact glass, and a toner image corresponding to the simulated original is formed on a photoreceptor drum after each copying operation. The detected density of the toner image then is compared with a reference level to drive a toner supply mechanism, if necessary, according to the comparison result to maintain optimum toner density.
In the aforementioned copying machine, the toner image density corresponding to the simulated original is detected after each copying operation, therefore, the image density can be maintained at a predetermined level according to the detection result. However, it is necessary to form a toner image of the simulated original for each copying operation. Furthermore, the toner used for forming this toner image is removed by a cleaning blade after the density detection, and thus more than the amount of toner necessary to provide copies of the original is consumed and also the cleaning ability of the cleaning blade is partially decreased.
It may be possible to solve the above inconveniences by the step of developing an electrostatic latent image directly to provide a toner image and another step of making the latent image disappear without developing a toner image, which steps are carried out periodically and selectively to reduce the frequency of toner image density detection. In this approach, a toner empty condition affects the density detection of the toner image thereafter, whereby density detection is not carried out for each copying operation, and toner corresponding to the original is consumed for each copying action during which no detecting is carried out, resulting in an earlier empty toner condition. Even if the toner empty condition was detected and toner was supplied accordingly, delayed detection of the density recovery again causes detection of an empty condition which results in halting operation of the copying machine to prevent degradation of the developing agent due to continued copying operations during a toner empty condition. The aforementioned problem becomes particularly significant if an original with a dominant dark portion is to be copied.
Another copying machine is known in which two developing devices are located near a photoreceptor drum and a different colored toner is stocked in each developing device. Operation is controlled to provide copies having the desired color. For developing electrostatic latent images formed on the photoreceptor drum by controlling the toner density in such copying machines, each developing device is provided with a contant feed device consisting of solenoids, one-way mechanisms, etc., a detection device for detecting toner density consisting of level sensors, coil inductors etc., and a toner supply device driven according to toner density detection signals.
In the aforementioned conventional copying machine, the two developing devices have different dimensions to accommodate different frequencies of use, etc.
Therefore, the attachment of such a control device with level sensors for the toner supply often may be difficult in a smaller developing device, even if possible in a larger one, and consequently a constant feed device with solenoids, one-way mechanisms, or the like is used.
As a result, if copying is carried out with a larger developing device, it is possible to obtain high-quality copies through precise density controls corresponding to various originals, but the precise density control is not achievable for copying with a smaller developing device and this results in degradation of the quality of the copies so obtained.